1. Technical Field
The present invention relates to a structure of a semiconductor device including an active region on a hetero junction of a nitride semiconductor.
2. Description of the Related Art
As a semiconductor device using a compound semiconductor, specifically, as a device for high power and high frequency, an High Electron Mobility Transistor (HEMT) device using such as GaN has been used. A schematic structure of a cross-section of an HEMT device 90 is shown in FIG. 4. In this figure, an electron transit layer 93 and an electron supply layer 94 are formed by an epitaxial growth method over a buffer layer 92 on a substrate 91. Here, the electron transit layer 93 is made of non-doped GaN, and the electron supply layer 94 is made of non-doped AlGaN (strictly, AlxGa1-xN, x=about 0.20). Here, a two-dimensional electron gas layer is formed on a portion of the electron transit layer in an interface between the electron transit layer 93 and the electron supply layer 94. The two-dimensional electron gas layer is formed between a source electrode 95 and a drain electrode 96, and electrical current flows through the two-dimensional electron gas layer. Here, a channel of the two-dimensional electron gas layer is controlled on-off by voltage applied to a gate electrode 97, and a switching operation of the transistor is performed. At this time, because a speed (mobility) of the electrons within the two-dimensional electron gas layer is extremely high, high speed operation is possible. Moreover, GaN has larger band gap energy than GaAs, etc., the HEMT device 90 has high insulation withstand voltage and can be perform high power operation. In this case, in order to obtain better amplification or switching characteristics, it is necessary to enhance an on-off ratio of the current flowing between the source electrode 95 and drain electrode 96, or a ratio of electrical resistance between the source electrode 95 and the drain electrodes 96 in the turning-off time to electrical resistance therebetween in the turning-on time. Incidentally, FIG. 4 merely shows the simplest structure of the HEMT device is shown FIG. 4. However, in practice, a contacting shape between the source or drain electrodes 95, 96 and the electron supply layer 94 or a shape around the gate electrode 97, etc., may be more optimized. Thus, a practical structure of the HEMT device may be different from the structure of the HEMT device in FIG. 4.
The electron transit layer 93 or the electron supply layer 94 is formed by the epitaxial growth over the substrate 91, and properties of the HEMT device 90 are considerably affected by a crystalline state of the electron transit layer 93 and the electron supply layer 94. Since the crystalline state and the manufacturing cost is considerably affected by the substrate 91, choosing what kind of the substrate 91 is important. For example, sapphire or semi-insulating SiC, etc., may be used as the substrate 91. However, because it is difficult to form the electron transit layer 93 (non-doped GaN) with better crystalline state directly on the material (wafer) such as sapphire or semi-insulating SiC, it is necessary that forming a buffer layer 92 made of other material between the substrate 91 and the electron transit layer 93.
Recently, as GaN wafer, n-GaN (n type GaN) wafer, which has a size easy to operate, can be obtain at a low cost and can be used as the substrate 91. For example, a fourth embodiment of JP-A-2009-12727 discloses a structure of the HEMT device in which wafer is used as a substrate 91. In this case, since the material of the substrate 91 is identical with that of the layer 93, it is comparatively easy to form non-doped GaN with the better crystalline state as the electron transit layer 93 on such the GaN substrate 91.
Further, in view of reducing on-resistance of the HEMT device 90, for example, FIG. 13 and FIG. 14 in JP-A-2006-216671 discloses that the through-hole electrode penetrating from the source electrode 95 to the substrate 91 is formed. Thus, a potential of the source electrode 95 intentionally comes to equal to the potential of the substrate 91 by the through-hole electrode. Accordingly, a reverse face electrode formed with a large size on the reverse face of the substrate 91 can be used as the source electrode. Therefore, as described in the paragraph [0046] of JP-A-2006-216671, it is not necessary to form a source electrode pad on the front surface (upper face) of the HEMT device 90, the entire area of the chip can be use effectively.
However, in the above-described arts, there is a problem with respect to the insulation withstand voltage or a leak current between the source electrode 95 and drain electrode 96, when high voltage is applied therebetween. That is, an insulation resistance between the source electrode 95 and drain electrode 96 becomes lower, or the leak current also flow even at pinch-off operation in the turning-on time, and then it may trouble in operation of the device.
As described above, the two-dimensional electron gas (channel), which is a main body of the current flowing in the turning-on time, pass through beneath the gate electrode 97 and is on-off controlled by the voltage applied to the gate electrode 97. However, electrical currents flow between the source electrode 95 and drain electrode 96 through paths other than the channel controlled by the gate voltage. The currents flowing through the other paths may become a main body of the leak current. As examples of the other paths, as indicated in FIG. 4 by arrows, there are paths passing through the buffer layer 92 and the substrate 91. If above described n-GaN wafer is used as the substrate 91, the n-GaN substrate 91 in itself is conductive, so that the leak current flows in the substrate 91.
In order to improve such a problem, it may be considered that MN layer, which is known to have larger band gap energy than that of GaN and has better insulation property, or non-doped AlGaN, which is mixed crystal of MN and GaN, is formed as the buffer layer 92 on n-GaN wafer (the substrate 91) and the electron transit layer 93 is formed on the buffer layer 92. However, in this case, it occurs great lattice mismatch (difference between lattice constants) between the buffer layer 92 made of AlN or AlGaN and the electron transit layer 93 made of GaN. Therefore, many crystal defects such as dislocations, etc., are generated in the interface between the buffer layer 92 and the electron transit layer 93, so that those crystal defects cause the electric conduction through thereof. Accordingly, the leak current (buffer leak) flow between the source electrode 95 and drain electrodes 96. This phenomenon also occurs when the sapphire or SiC, etc., is used as the substrate.
This problem is not limited to only the HEMT device also other devices, which include an substrate including GaN and an active layer having a hetero structure on the substrate, and in which electric current flows in a lateral direction (i.e. a direction parallel to the surface of the substrate) for operating the device. Examples of such devices may be Metal Oxide Semiconductor Field Effect Transistor (MOSFET) or Schottky Barrier Diode (SBD), etc.
Accordingly, in a prior art, it is difficult to manufacture the device in which the buffer leak is reduced on the substrate containing GaN.
The present invention is made with considering the above problem, and provides the device solved the problem.